Nsumable electrode method and apparatus for providing single crystal castings using a co

ABSTRACT

Apparatus and method by which to produce single crystal articles using a ceramic mold and consumable arc melting technique with cooling from a chill plate at the base of the apparatus and with the ceramic mold limiting the heat loss laterally from the molten metal.

[ Sept. 18, 1973 4/1964 Veil 5/1957 Delano............

10/1971 Simmons......... 8/1956 Laird et al. 9/1954 GreatBritain........................

ABSTRACT 7 Claims, 1 Drawing Figure FOREIGN PATENTS OR APPLICATIONSPrimary ExaminerJ. Spencer Overholser Assistant ExaminerJohn E.'RoethelAttorney-Charles A. Warren Apparatus and method by which to producesingle crystal articles using a ceramic mold and consumable arc meltingtechnique with cooling from a chill plate at the base of the apparatusand with the ceramic mold limiting the heat loss laterally from themolten metal.

, MOTOR atent 1y. Sink, Milwaukie, Oreg.

PROVIDING SINGLE CRYSTAL CASTlNGS USING A CONSUMABLE ELECTRODEInventors: Robert B. Barrow, Cheshire,

Matthew F. Smith, Shelton, both of Conngl arry East Hartford, Conn.

Aug. 30, 1971 Appl. No.: 176,215 I Related US. Application Data [63]Continuation-impart of Sen-No. 803,539, March 3,

' 1969, abandoned.

Int. [58] Field of Search..................

References Cited UNITED STATES PATENTS 5/1966Chandley............................

United States Barrow et a1.

[ METHOD AND APPARATUS FOR [73] Assignee: United Aircraft Corporation,

[22] Filed:

METHOD AND APPARATUS FOR PROVIDING SINGLE CRYSTAL CASTINGS USING ACONSUMABLE ELECTRODE This is a continuation-in-part application of Ser.No. 803,539 filed Mar. 3, 1969 now abandoned for SIN- GLE CRYSTALCASTINGS.

BACKGROUND OF THE INVENTION SUMMARY OF THE INVENTION One feature of thisinvention is the formation of single crystal articles by a consumablearc melting technique combined with a procedurefor controlling thesolidification rate thereby to produce a fine-grained dendriticstructure. Another feature is an apparatus by which to produce such adendritic microstructure. Another feature is an apparatus and process bywhich to obtain both the desired'dendritic structureand also a desiredorientation of the dendritic structure such, for example, as an [001]orientation along the longitudinal axis of the article.

According to the invention the apparatus includes a chill plate by whichto start and continue the cooling of the melt at the desired rate onwhich is positioned a crystalline seed with the dendritic orientationthe same as that desired in the cast article. Extending upwardly fromthe chill plate is a relatively thick mold or shell of ceramic materialin which the article is cast and the effective thickness of the mold maybe increased by an annulus of granular ceramic material. This annulus issupported by a metallic crucible sleeve or cylinder which surrounds themold and may, if necessary, have cooling means associated with it toprevent melting of the sleeve during the casting operation. A consumableelectrode of the alloy to form the casting is supported above the moldand projects downwardly into the mold to contact the seed. Electricalenergy supplied to the electrode and seed form an are which melts theelectrode to form a molten body of alloy which crystallizes upward fromtheseed as a single crystal to form the cast article by the downwardremoval of heat through the seed into thechill plate. Insulatingmaterial around the seed protects the chill plate from the heat of thearc.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is a verticalcross-sectional view through an apparatus embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, themethod may be carried out by the apparatus shown which includes a chillplate or crucible bottom to the center of which is secured asingle-crystal seed crystal 11 having the desired orientation of thedendritic growth to produce the selected crystalline orientation in thecompleted cast article. The mold for the cast article is a relativelythick ceramic sleeve 12 on the order of as much as one-fourth inch andhaving its lower end resting on the chill plate 10 and surrounding theseed crystal. This ceramic sleeve is surrounded in spaced relation by acrucible sleeve 14 solidly secured at its base to the chill plate as bybolts 15.

The space between sleeve 12 and the crucible sleeve is filled with aceramic grog or a granular ceramic l6 and the thickness of this spacemay be as much as from three-fourths to nearly 3 inches, thereby givingan effective heat resistant thickness within the crucible sleeve of from1 to 3.inches preferably approaching the greater thickness. A heatingcoil 17 is embedded in the granular ceramic in close relation to or inengagement with the ceramic sleeve 12 and this coil serves to assist inraising the temperature of this sleeve to a temperature slightly abovethe melting temperature of the alloy being cast. The radiation from thearc and from the surface of the molten alloy supplements the action ofthe heating coil in this respect as will be described. This cruciblesleeve 14 may in turn be surrounded by a cooling jacket not shownalthough this is not essential and serves to keep the sleeve 14 fromoverheating. The space between the seed crystal and the walls of theceramic sleeve 12 may be filled with an insulating'ceramic material 18to prevent damage to the face of the chill plate in operating thedevice.

The seed crystal is preferably as large as possible in a horizontaldimension to cover as much area of the.

chill plate surface enclosed within the end of the sleeve 12 as the heatconduction from the melt to the chill plates is through this crystal andthe greater the area of the crystal the greater the amount of heat thatcan be conducted away for each interval of time. This seed crystal will,however, have the desired dendritic orientation such that itwill fitreadily and securely in a recess 19 in the chill plate, The crystal isalso preferably relatively thin but of such a thickness as will bedescribed later, that the arc melting of the surface at the start of thecasting operation will not melt to an extent to involve the chill plate.

A consumable-bar 20 of the alloy to'be cast is positioned within thesleeve 12 and is carried by any of the conventional electrode feedingdevices by which an arc may be struck and maintained in the melting ofthe electrode within the sleeve. One type of well known consumable arcmelting furnace having this type of feed equipment is the Heraeus L200 Hfurnace. For the purpose of the present invention, it is sufiicient tonote that the bar 20 is mounted on a feed rack 21 that is moved by agear 22 to advance the electrode into or out of the sleeve 12 at theappropriate rate to maintain the desired melting of the ingot. It willbe understood that the arc is formed by an electrical arc, the energybeing supplied from a suitable source 24 with power leads 26, as shown,to the electrode and to the crucible. Obviously, a suitable control 28of conventional construction maintains the necessary power supply to thearc and also serves to control the electrode feed rate.

The sleeve 12 being ceramic acts as a heat barrier to the molten alloyin the sleeve so that, as the electrode melts to form a pool within thesleeve, the effective heat removal by which to cause solidification ofthe molten material occurs axially downwardly into and through the chillplate at the base. The solidification front thus advances upwardly fromthe crystal so that the solidifying metal becomes a single crystalextension of the crystalline lattice of the seed crystal and thiscrystalline orientation is the same as that of the seed crystal.Superheated molten metal is continually supplied to the pool of moltenalloy above the liquid-solid interface and a very steep temperaturegradient is maintained in the solidification zone. With the verticallydownwardly removal of heat through the solidified metal and into thechill plate the necessary upwardly moving solidification front forsingle crystal formation is maintained. This arrangement produces a veryfast solidification rate producing a fine single crystal dendriticstructure.

When the are for melting the bar is originally struck between theelectrode and the seed crystal, the latter is melted back to some extentbut a substantial portion remains solid being in direct contact with thechill I plate. The sleeve 12 is preferably heated to a temperature abovethe melting temperature of the alloy being cast prior to or at thebeginning of the casting operation. Thus, the heating coil 17 isenergized for the purpose of heating this sleeve 12 and the radiationfrom the arc and from the molten pool of alloy in conjunction serves toheat the wall surface of the sleeve to the desired temperature.Preferably the heating coil will have been energized in advance ofstriking the arc in order to have the mold nearly up to the requiredtemperature prior to the start of the melting and casting operation sothat shortly after the arc is struck, the portion of the exposed walladjacent therto will reach the appropriate temperature; It is importantthat the exposed wall surface of the mold at and above thesolidification zone be at this temperature to avoid spurious nucleationon this wall surface which would cause the continued growth ofextraneous grains along'the surface of the single crystal casting.

The portion of the sleeve immediately above the level of the pool isexposed directly to the arc and with no significant heat loss throughthe sleeves, this portion of the wall surface of the sleeve is quicklyraised to a temperature above the melting temperature of the alloy andthis arc-heated area of the wall moves upwardly as the arc is caused tomove up with an increase in height of the pool surface during thecasting operation.

Being ceramic, the sleeve acts as a heat barrier and no removal. of heatfrom the solidifying alloy occurs in a lateral direction into thesleeve. Thus, the desired heat removal in a vertical downward directionfrom the solidifying alloy is assured by this are heating of the sleeveas the bar 19 of alloy is being arc melted. The are is always above thesurface of the molten material within the sleeve and the latter is thusdirectly exposed to the radiant heat of the are. It will be understoodthat the rate of melting and thus of solidification of the singlecrystal is a function of the power input and the effectiveness of thechill with the preheating of the sleeve by the are assisting inproducing a high thermal gradient. The production of singlecrystal partsby the use of a chill and a steep thermal gradient in the mold isdescribed and claimed in Piearcey US. Pat. 3,494,709.

For example, in producing a single-crystal ingot of l '16 inches indiameter, the chill plate was a copper plate with cooling passagestherein and the cooling flow of water was set at 8 gallons per minute.The electrical input for the arc was 220 amps at 21 volts and theelectrode feed rate was substantially a inch per minute, the

electrode being 0.75 square inch in cross section. This ingot was castfrom Mar M 200 alloy and the seed crystal was of the-same alloy. Theseed crystal was produced by vacuum induction casting.

The resulting ingot was found to have the desired microstructure and areduced microporosity. The high solidification rate obtainable byconsumable arc melting produces closer dendritic arm spacing, a smalleutectic phase and a finer distribution of smaller MC carbides all ofwhich provide a better casting.

We claim:

1. In the manufacture of a single crystal article, the steps of mountingan oriented single crystal seed on a chill block,

providing a ceramic shell extending upwardly from the chill plate toserve as a mold and as a heat barrrer,

striking an arc between said seed and a consumable electrode of thematerial for the article to cause partial melting of the seed andmelting of the electrode,

heating the ceramic shell at least in part by radiant heat from the areto minimize heat loss through the mold walls, solidifying the seed andthe molten metal of the electrode upwardly from the seed by heatconduction from the seed and molten metal into the chill plate,

continuing to melt the electrode ahead of the liquidsolid interface ofthe solidifying material, and

maintaining by heat from the are a high temperature in the inner surfaceof the shell at and above the level of the liquid-solid interfacethereby to prevent nucleation on the shell.

2. The process of claim 1 in which the shell is exposed directly to theradiant heat of the arc and the wall surface of the shell is heated to atemperature above the melting temperature of the alloy.

3. Apparatus for casting oriented single crystal articles by consumablearc melting, including a cooled chill plate,

a cylinder above and resting on the chill plate,

an oriented single crystal seed crystal on the chill plate, 7

a ceramic mold within the cylinder of sufficient thickness to form aheat barrier to the heat from the arc in which the article is formed,said seed crystal being in the base of said mold,

means positioned close to the mold between the mold and cylinder forheating the mold, and

a consumable electrode positionable within the mold.

4. The apparatus of claim 3 inwhich the chill plate is protected fromthe are by a ceramic material surrounding the seed crystal and coveringthe chill plate.

5. The'apparatus of claim 3 including means for supplying electricalenergy from the electrode to the seed crystal to cause arc melting ofthe electrode and for heating of the wall of the mold.

6. Apparatus for casting an oriented single crystal ingot including achill plate,

an oriented single crystal seed secured to the plate,

a crucible surrounding the seed and projecting upwardly from the chillplate,

a thick ceramic mold within and spaced from the crucible,

means externally of the mold and between the mold trode within the moldand for heating the inner wall and crucible for heating the crucible, ofthe mold. a consumable electrode movable into the crucible to 7.Apparatus as in claim 6 including agranular cecontact with the seed, andramic filling the space between the crucible and the means for supplyingelectrical energy between said 5 mold.

electrode and seed for causing melting of the elecmy I UNITED STATESPATENT OFFICE CERTIFICATE OF RRCTEIQN Patent No. 3,759,310 Y DatedSeptember 18, 1973 Inventor(s) ROBERT B. BARROW ET AL It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

In the Title the word "Providing" should read --Producing-- Signed andsealed this 26th day of March 1974.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. c. MARSHALL DANN Attesting Officer Commissioner ofPatents

1. In the manufacture of a single crystal article, the Steps of mountingan oriented single crystal seed on a chill block, providing a ceramicshell extending upwardly from the chill plate to serve as a mold and asa heat barrier, striking an arc between said seed and a consumableelectrode of the material for the article to cause partial melting ofthe seed and melting of the electrode, heating the ceramic shell atleast in part by radiant heat from the arc to minimize heat loss throughthe mold walls, solidifying the seed and the molten metal of theelectrode upwardly from the seed by heat conduction from the seed andmolten metal into the chill plate, continuing to melt the electrodeahead of the liquid-solid interface of the solidifying material, andmaintaining by heat from the arc a high temperature in the inner surfaceof the shell at and above the level of the liquid-solid interfacethereby to prevent nucleation on the shell.
 2. The process of claim 1 inwhich the shell is exposed directly to the radiant heat of the arc andthe wall surface of the shell is heated to a temperature above themelting temperature of the alloy.
 3. Apparatus for casting orientedsingle crystal articles by consumable arc melting, including a cooledchill plate, a cylinder above and resting on the chill plate, anoriented single crystal seed crystal on the chill plate, a ceramic moldwithin the cylinder of sufficient thickness to form a heat barrier tothe heat from the arc in which the article is formed, said seed crystalbeing in the base of said mold, means positioned close to the moldbetween the mold and cylinder for heating the mold, and a consumableelectrode positionable within the mold.
 4. The apparatus of claim 3 inwhich the chill plate is protected from the arc by a ceramic materialsurrounding the seed crystal and covering the chill plate.
 5. Theapparatus of claim 3 including means for supplying electrical energyfrom the electrode to the seed crystal to cause arc melting of theelectrode and for heating of the wall of the mold.
 6. Apparatus forcasting an oriented single crystal ingot including a chill plate, anoriented single crystal seed secured to the plate, a cruciblesurrounding the seed and projecting upwardly from the chill plate, athick ceramic mold within and spaced from the crucible, means externallyof the mold and between the mold and crucible for heating the crucible,a consumable electrode movable into the crucible to contact with theseed, and means for supplying electrical energy between said electrodeand seed for causing melting of the electrode within the mold and forheating the inner wall of the mold.
 7. Apparatus as in claim 6 includinga granular ceramic filling the space between the crucible and the mold.